The present invention relates to a color registration system for a multi-color printing press. More specifically, it relates to a method and apparatus for sensing and maintaining the registration of each of the color printers utilized in conventional multi-color printing presses.
Three- and four-color process printing is closely allied with color photography. The fundamentals of color reproduction are much the same for all the printing processes. Just as a halftone printed in black on white paper is an optical illusion giving the observer the impression that he is seeing the various gradations of tone in the original photograph, a colored halftone gives the illusion that a wide range of colors is present. Actually, only three colors are used (black is added in the more commonly used four-color process). The halftone screens for each color are printed at differing angles, so that the dots fall one alongside another and overlap to form combinations of many colors. Inks for process printing are transparent tones of red, blue and yellow.
Since three plates (four for four-color process) must be obtained in order to print the proportionate parts of the different colors, first steps involve breaking down a colored original into three (or four) separate photographic images. These are termed separation negatives.
In one conventional method of producing separation negatives, the colored original is positioned before the lens of a large copying camera. Over the lens is placed an orange-red filter which allows light rays of that color only to pass. Thus, the red portions of the original are represented by tones of gray in the negative. In like manner, another piece of film is placed in the camera and another exposure made with a green filter over the lens. This negative now contains the green portion of the original. Again, a third piece of film is exposed through a deep-blue filter, this negative giving a record of the blue areas in the original.
Although it is theoretically possible to print the full range of tones using only the three process colors, operators usually include a black printer to add detail and contrast to the printed reproduction. To make the separation negative for the black printer, the same general procedure used for the other separation negatives is followed.
From each of the separation negatives, a positive print is made. A positive represents tonal values reversed from those in the negative, so the positive made from the red-filter negative will represent all colors except red, in other words, minus red. White light minus red leaves blue-green, the color in which this positive should be printed. In process work this color is called cyan. The green-filter negative produces a positive which must be reproduced in minus green, which is bluish-red or magenta. The blue-filter negative in turn gives a positive which must be printed as minus blue, or yellow. When the three positives are brought together one over the other in exact alignment, the original subject is recreated.
The steps followed from the breaking down of a colored original into separation negatives to the generation of combined positive prints vary according to the particular graphic arts process used. For example, in offset lithography the separation negatives are each photographed through a halftone screen to give a screened positive from which three (or four) printing plates are made. These plates are printed, superimposed in register, for the proper color of the final reproduction.
Thus, although there are numerous types of designs of printing presses, most share the problem of having to print multiple single-color images in the same area in order to achieve the final, multi-colored result. Accordingly, for a high quality final product to be achieved these multiple single-color images must be accurately located, or registered, with respect to one another. The tolerable error with respect to ideal register depends on the particular application, but is often in the range of 0.001".
In order to achieve proper register, the printing press operator must make appropriate mechanical adjustments of the printing press. Moreover, adjustments must be made to achieve registration in both the longitudinal (i.e., in the direction of paper motion) and lateral dimensions. Once initial registration is achieved in the make-ready phase of press operation, the relative registration of the separate color images is likely to change as the printing press runs. Typical causes of this misregistration include fluctuations in the speed of operation, changes in ambient conditions and varying characteristics of the print substrate.
It is apparent that once the printing press is running, the effect of misregistration is to degrade the sharpness and color quality of the final printed image. Accordingly, it is of advantage to be able to determine the degree of registration quite often during the running of the press, in order that slight misregistration be sensed and corrected before it becomes intolerable. Once information as to the relative registration of the single-color images is available, the printing press can be adjusted, either manually or automatically, to correct any misregistration.
Known color registration sensing methods utilize sensors that operate by the measurement of register marks printed along with the color images. This class of sensors embodies the basic principle that longitudinal registration errors result in a change in the relative times at which the register marks are measured by the sensor. In one known implementation, lateral registration is determined by measuring the relative timing of two kinds of register marks, one perpendicular to and one skew to the direction of paper motion. Both marks must be measured before lateral registration can be determined, since the timing of the skew mark is dependent upon both the longitudinal and lateral registration.
There are several limitations and drawbacks associated with the knowncolor registration measurement methods. Specifically, they do not provide for independent measurement of registration in two dimensions. Lateral registration measurement requires that longitudinal registration first be measured, and its value used to compute lateral registration. Thus, errors in measuring longitudinal registration will affect the determination of lateral registration.
Moreover, they require knowledge of paper speed, since they are based on measuring the relative timing of register marks. Errors in determining paper speed can therefore introduce errors into the measurement of registration.
And finally, they can be adversely affected by the effects of fill-in and slur on the printed registration marks, and do not compensate for such adverse effects.
It is accordingly a general object of the present invention to provide a method and apparatus for sensing and maintaining the registration of each of the color printers utilized in conventional multi-color printing presses. More specifically, it is an object of the invention to overcome the aforementioned limitations and drawbacks associated with the known techniques.
It is a particular object of the invention to provide a color registration sensing method that independently senses longitudinal and lateral registration.
It is another object of the invention to provide a color registration sensing method that does not require knowledge of paper speed.
It is still another object of the invention to provide a color registration sensing method that compensates for the potentially adverse effects of fill-in and slur on the printed register marks.
It is a further object of the invention to provide a color registration sensing method whereby the sensitivity of the registration measurement and the range of registration that can be sensed can be changed through the choice of register marks.
Other objects will be apparent in the following detailed description and the practice of the invention.